Directly Revealing the Structure-Property Correlation in Na -Doped Cathode Materials

Abstract

The introduction of Na+ is considered as an effective way to improve the performance of Ni-rich cathode materials. However, the direct structure-property correlation for Na+ doped NCM-based cathode materials remain unclear, due to the difficulty of local and accurate structural characterization for light elements such as Li and Na. Moreover, there is the complexity of the modeling for the whole Li ion battery system. To tackle the above-mentioned issues, we prepared Na+-doped LiNi0.6Co0.2Mn0.2O2 (Na-NCM622) material. The crystal structure change and lattice distortion with picometers precision of Na+-doped material is revealed by Cs-corrected scanning transmission electron microscopy (STEM). Density functional theory (DFT) and the recently proposed electrochemical model, i.e., modified Planck-Nernst-Poisson coupled Frumkin-Butler-Volmer (MPNP-FBV), has been applied to reveal correlations between activation energy and charge transfer resistance at multiscale. It is shown that Na+ doping can reduce the activation energy barrier from ΔG = 1.10 eV to 1.05 eV, resulting in a reduction of the interfacial resistance from 297 Ω to 134 Ω. Consequently, Na-NCM622 cathode delivers a superior capacity retention of 90.8% after 100 cycles compared to pristine NCM622. Our results demonstrate that the kinetics of Li+ diffusion and electrochemical reaction can be enhanced by Na+ doping the cathode material.

Keywords: LiNi0.6Mn0.2Co0.2O2, Na+-doping, Transmission electron microscopy, Migration energy barrier, Charge transfer resistance